The present invention relates to an optical recording apparatus for recording information on a recording medium with use of high density energy beams such as a laser beam.
A rewritable optical recording medium is capable of being recorded at a density much higher than that of a rewritable magnetic recording medium. At present, the on-line capacity of the whole optical recording apparatus, that is, the recording capacity allowable for a simultaneous access, is about the same as that of a magnetic hard disk apparatus. There is therefore a demanded for an optical recording apparatus having a larger on-line capacity.
Heretofore, separately from the development of recording materials, there has been proposed a method in which recorded marks are recorded as closely as possible on the basis of the current bit-by-bit recording method using heat mode recording materials so as to increase the recording density in a surface. For example, a method is known (U.S. Pat. No. 4,569,038) in which the shape of a groove for guiding the light is so designed as to reduce the influence of the reflected light from adjacent tracks, thereby making the pitch of the recording tracks small. With this method, however, the track pitch can be limited to about half of the present pitch at best, and it is impossible to decrease the pitch much more because of an increase of cross-talk. Namely, the capacity can be increased only twice at best.
As another method for increasing the recording density using the conventional recording medium, there is known in general a method in which the wavelength of a laser beam, 80 nm or so at present, is shortened to 70 nm, 60 nm, . . . , for example, or the numerical aperture (N.A.) of an objective lens, about 0.5 at present, is increased to 0.6, 0.7, . . . , for example, so that the size of the recorded marks is reduced to thereby increase the recording density. However, the former method should rely on development of a high power short wavelength semiconductor laser, while the latter method has a problem in that the existing optical recording technique (such as focusing servo), inclusive of the structure of a recording medium and the structure of a recording head, is not applicable as it is in respect of the working distance of the objective lens as well as a considerable decrease in the depth of focus.
In the magnetic hard disk apparatus, the number of recording surfaces available for the simultaneous access is increased by using a recording body comprising a plurality of stacked magnetic recording media and magnetic heads arranged for the respective surfaces, thereby increasing the on-line capacity. In the field of the optical recording apparatus as well, an apparatus of so-called jukebox type has already been commercialized in which a recording medium is taken out according to demand from a stocker storing a plurality of recording media. In this apparatus, however, the on-line capacity is limited by the capacity of a single recording medium, resulting in a problem in that it takes time to achieve access. Further, the apparatus is too large to be built in a personal computer, for example. Namely, assuming that the stacked structure is formed by using the existing optical recording heads and optical recording media, it is difficult to reduce the size of the apparatus, resulting in impracticability. In other words, the conventional optical recording apparatus has a problem in that since the size of the optical recording head is much greater than that of the magnetic recording head, the thickness (height) of the apparatus is inevitably increased to a great extent merely by stacking simply the conventional optical recording media and optical recording heads. Namely, it is difficult to achieve a large capacity due to stacking like the magnetic disk apparatus only by applying the conventional techniques.